Fix route flags update during RTM_CHANGE.
Nexthop lookup was not consireding rt_flags when doing
structure comparison, which lead to an original nexthop
selection when changing flags. Fix the case by adding
rt_flags field into comparison and rearranging nhop_priv
fields to allow for efficient matching.
Fix route change X/Y flags case - recent changes
disallowed specifying RTF_GATEWAY flag without actual gateway.
It turns out, route(8) fills in RTF_GATEWAY by default, unless
-interface flag is specified. Fix regression by clearing
RTF_GATEWAY flag instead of failing.
Fix route flag reporting in RTM_CHANGE messages by explicitly
updating rtm_flags after operation competion.
Add IPv4/IPv6 tests for flag-only route changes.